Atomic scale diffusion in complex systems from first principles

Transport of point defects controls a variety of materials process such as precipitation, segregation of solutes to grain boundaries and surfaces, and macroscopic properties such as corrosion resistance and ionic conductivity. Therefore, a quantitative prediction of atomic scale transport is crucial to development of new alloys. First principles calculations coupled with advanced diffusion models can accurately predict atomic scale transport mechanisms of point defects in solids. In this work, we examine transport of six solutes - Sn, Cr, Fe, Be, Al and Ni in HCP Zr, and the transport of oxygen vacancies in LaGaO$_3$. Zirconium alloys are used as nuclear fuel cladding materials for light water power reactors and understanding point defect diffusion in Zr will provide a step forward for developing oxidation tolerant alloys. We accurately model the vacancy metastable states observed in HCP Zr and for the first time examine the effect of these states on solute transport. Our results show that Sn and Al diffuse via vacancy mediated mechanism while Cr, Fe, Be and Ni diffuse via the interstitial mechanism at equilibrium. The drag ratios of Cr, Fe, Be and Ni are positive which suggests that non-equilibrium vacancy fluxes could drag these solutes. By combining interstitial and vacancy mediated diffusivities, we demonstrate that supersaturated vacancy concentrations slow down the interstitial diffusion while accelerating the vacancy mediated diffusion. In recent years, LaGaO$_3$ has attracted considerable interest for applications in solid oxide fuel due to high oxygen ion mobilities but the atomic scale diffusion mechanism of oxygen vacancies is not well understood. We examine the atomic scale migration of oxygen vacancies in LaGaO$_3$ and study the effect of strain on the diffusivities. We find that O vacancy diffusion is nearly isotropic in undoped LaGaO$_3$ and strains up to 2\% can accelerate the diffusivity by two orders of magnitude which could help reduce operating temperatures of the fuel cells. Strong attractive Sr-vacancy and vacancy-vacancy interactions lead to formation of superbasins which trap the vacancy at lower concentrations. However, at sufficiently high concentrations, these superbasins could overlap and lead to fast diffusion via percolation.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste